Semiconductor lasers as discrete devices often enjoy the luxury of having p-side and n-side electrodes on opposite sides of the semiconductor device. One electrode can be formed on a doped substrate and the other on the opposite surface of the semiconductor device on which the epitaxially grown and treated layers are formed.
It is often useful to produce laser devices in which both the p-side and n-side electrodes are on the same surface of the semiconductor device, usually the surface opposite the substrate which carries the epitaxially grown and otherwise treated semiconductor layers. Lasers with both electrodes on a single surface are often called "planar type" lasers. That term is typically taken to mean that the electrodes are formed on the same surface of the laser, and also at about the same "level" with respect to the substrate. Among the reasons for forming devices of this type is the possibility of integrating the laser with other electronic circuitry.
FIG. 4 shows in cross section the structure of a multi-quantum well laser of the planar type as an example of a prior art structure. The FIG. 4 laser device is based on a semi-insulative GaAs substrate 1. Formed on the GaAs substrate in sequence are a p-type AlGaAs lower cladding layer 21, a multi-quantum well active layer 3, an n-type AlGaAs upper cladding layer 4, and an n-type GaAs contact layer 5. P-type diffusion regions 9, typically Zn diffusion regions, are formed to disorder the portions of the multi-quantum well active layer 3 which they invade, and to leave a non-disordered central stripe portion 30 of the active layer to serve as a light-emitting region. The p-type diffusion regions 9 invert the conductivity of the invaded portions of the n-type AlGaAs upper cladding layer 4 and n-type GaAs contact layer 5. Thus, p-side electrodes 12 formed over the p-type diffusion regions 9 serve as the positive electrode for the device, whereas an n-side electrode 11 formed over the active stripe region 30 forms a connection for the n electrode. The contact region 5 is divided in about the area of the active region 30 to electrically disconnect the material underlying the p-side and n-side electrodes.
In production, there are successively grown on a semi-insulative GaAs substrate, by conventional epitaxial growth processes, the p-type AlGaAs lower cladding layer 21, multi-quantum well active layer 3, n-type AlGaAs upper cladding layer 4, and n-type GaAs contact layer 5. Following the epitaxial growth process, the upper surface of the device is masked, a source of Zn impurities is formed on the upper surface of the device, and the device is annealed to form p-type diffusion regions 9. Diffusion is conducted until the diffusion front reaches the substrate and defines a narrow non-disordered active stripe 30 in the active layer, bounded by diffusion-induced disordered regions. As suggested in the drawings, diffusion proceeds both downwardly and laterally, and the diffusion conditions are controlled to define a very narrow stripe 30 necessary to assure single transverse mode oscillation. The stripe width 30 is typically on the order of 2 microns or less, suggesting the precision required of the diffusion operation.
Following the formation of the p-type diffusion regions 9, the diffusion source and mask are removed, the n-type GaAs contact layer 5 is etched to separate the central portion from the end portions of that layer, and n-side and p-side electrodes 11, 12, respectively, are produced by conventional plating and lift-off techniques. It will be appreciated that the size and positioning of the n-side electrode 11 must be controlled very accurately since it cannot extend beyond the relatively narrow central stripe 30 without producing defective devices.
In operation, when a bias source is connected, positive terminal to p-side electrodes 12 and the negative terminal to n-side electrode 11, a p-n junction across the non-disordered central stripe 30 will be forward biased to cause current flow. Holes will flow from the positive terminal of the bias source through the p-side electrode 12 and the disordered region 9. Electrons will flow through the n-type electrode 11, the n-type contact layer 5 and the n-type upper cladding layer 4. The diffusion voltage of the multi-quantum well layer 3 is lower than the diffusion voltage of the p-n junctions formed in the upper and lower cladding layers 4, 21, respectively, and therefore carriers will be injected into the non-disordered central stripe 30 of the multi-quantum well active layer 3. As a result of carrier injection into the non-disordered stripe 30, light emission occurs. Since the refractive index of the stripe 30 is higher than that of the disordered regions of the active layer 3 and higher than that of the upper and lower cladding layers 21, 4, light is substantially confined to the narrow stripe 30. Since the width of that stripe is made relatively small, such as on the order of 2 microns, oscillation occurs in a stable single transverse mode, and the laser device itself exhibits a relatively low threshold current. It is a feature of the FIG. 4 embodiment that both p- and n-side electrodes 12, 11 are produced on the same side of the device and with a small or insubstantial step difference, thereby making the device suitable for integration.
The device of FIG. 4, however, suffers from a substantial drawback in balancing the need for a very small stripe width against processability of the device. Whereas single transverse mode operation demands a relatively narrow stripe width, the structure of the device demands an electrode which is no wider than the stripe width, requiring significant precision in forming the n-side electrode 11 over the central stripe 30 in order to prevent the n-side electrode from overlying or contacting the relatively larger p-side electrode areas. Since the structure of the laser limits the width of the electrode 11, the resistance of the n-side electrode can become quite high, thereby resulting in a significant limitation in use of such a diode in a continuous oscillation mode. Thus, the prior art device of FIG. 4 possesses not only processing difficulties which can tend to limit yield, but also operational difficulties in that the operating mode must take into account the relatively higher contact resistance which goes hand-in-hand with the narrow electrode width.